Some say it is folklore other not. But the story is that one in five miles of interstate are built straight and flat to serve as an emergency runway in times of war. Rather that is true or not I don't know. I do know that part of the big push for the interstate system from Eisenhower was to be able to move troops quickly from place to place. This purpose, I beleive was the reason for the Autobahn.
That was the intent, but there was a SNAFU in the shipping department. It turned out to be cheaper to reshape the terrain throughout the country to fit the roadway sections delivered, which is why Nebraska is so flat, and why there's very few 1-mile long tangents throughout most hilly and mountainous areas.
